High Efficiency and Power Transfer in  Wireless Power Magnetic Resonators

ABSTRACT

Techniques for wireless power transmission at levels that induce high power transfer and/or high efficiency of coupling.

This application claims priority from provisional application No.60/973,166, filed Sep. 17, 2007, the entire contents of which disclosureis herewith incorporated by reference.

BACKGROUND

It is desirable to transfer electrical energy from a source to adestination without the use of wires to guide the electromagneticfields. A difficulty of previous attempts has been low efficiencytogether with an inadequate amount of delivered power.

Our previous applications and provisional applications, including, butnot limited to, U.S. patent application Ser. No. 12/018,069, filed Jan.22, 2008, entitled “Wireless Apparatus and Methods”, the entire contentsof the disclosure of which is herewith incorporated by reference,describe wireless transfer of power.

The system can use transmit and receiving antennas that are preferablyresonant antennas, which are substantially resonant, e.g., within 5-10%of resonance, 15% of resonance, or 20% of resonance. The antenna(s) arepreferably of a small size to allow it to fit into a mobile, handhelddevice where the available space for the antenna may be limited. Anefficient power transfer may be carried out between two antennas bystoring energy in the near field of the transmitting antenna, ratherthan sending the energy into free space in the form of a travellingelectromagnetic wave. Antennas with high quality factors can be used.Two high-Q antennas are placed such that they react similarly to aloosely coupled transformer, with one antenna inducing power into theother. The antennas preferably have Qs that are greater than 1000.

SUMMARY

The present application describes transfer of energy from a power sourceto a power destination via electromagnetic field coupling with highefficiency and/or high power. Embodiments describe operations and actualefficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects will now be described in detail with referenceto the accompanying drawings, wherein:

FIG. 1 shows a block diagram of a magnetic wave based wireless powertransmission system;

FIG. 2 illustrates a transmitter block diagram including amplifiercoupling loop and antennas;

FIGS. 3 illustrates a receiver block diagram including coupling loop,receiver and trends at tuning element;

FIG. 4 illustrates the received power over distance;

FIG. 5 illustrates the maximum transferable power over distance;

FIG. 6 illustrates the transfer efficiency over distance;

FIG. 7 illustrates the efficiency normalized to antenna diameter; and

FIG. 8 illustrates an antenna measurement setup.

DETAILED DESCRIPTION

A basic embodiment is shown in FIG. 1. A power transmitter assembly 100receives power from a source, for example, an AC plug 102. A frequencygenerator 104 is used to couple the energy to an antenna 110, here aresonant antenna. The antenna 110 includes an inductive loop 111, whichis inductively coupled to a high Q resonant antenna part 112. Theresonant antenna includes a number N of coil loops 113 each loop havinga radius R_(A). A capacitor 114, here shown as a variable capacitor, isin series with the coil 113, forming a resonant loop. In the embodiment,the capacitor is a totally separate structure from the coil, but incertain embodiments, the self capacitance of the wire forming the coilcan form the capacitance 114.

The frequency generator 104 can be preferably tuned to the antenna 110,and also selected for FCC compliance.

This embodiment uses a multidirectional antenna. 115 shows the energy asoutput in all directions. The antenna 100 is non-radiative, in the sensethat much of the output of the antenna is not electromagnetic radiatingenergy, but is rather a magnetic field which is more stationary. Ofcourse, part of the output from the antenna will in fact radiate.

Another embodiment may use a radiative antenna.

A receiver 150 includes a receiving antenna 155 placed a distance D awayfrom the transmitting antenna 110. The receiving antenna is similarly ahigh Q resonant coil antenna 151 having a coil part and capacitor,coupled to an inductive coupling loop 152. The output of the couplingloop 152 is rectified in a rectifier 160, and applied to a load. Thatload can be any type of load, for example a resistive load such as alight bulb, or an electronic device load such as an electricalappliance, a computer, a rechargeable battery, a music player or anautomobile.

The energy can be transferred through either electrical field couplingor magnetic field coupling, although magnetic field coupling ispredominantly described herein as an embodiment.

Electrical field coupling provides an inductively loaded electricaldipole that is an open capacitor or dielectric disk. Extraneous objectsmay provide a relatively strong influence on electric field coupling.Magnetic field coupling may be preferred, since extraneous objects in amagnetic field have the same magnetic properties as “empty” space.

The embodiment describes a magnetic field coupling using a capacitivelyloaded magnetic dipole. Such a dipole is formed of a wire loop formingat least one loop or turn of a coil, in series with a capacitor thatelectrically loads the antenna into a resonant state.

Our previous applications have described the advantages of single turnloops being used as the resonators. The present application describeshow two different single turn loops can be used to produce significantlyincreased range in a wireless power transmission system.

In the embodiment, a test was carried out using the test setup shown inFIG. 8. The transmitter 801 is a 45 cm diameter, 6 mm wire loop. Thereceiver is formed of a 40 cm×30 mm copper loop. It is noted thatusually the receiver antenna should be smaller for purposes ofpackaging. As explained further herein, the test results are whollyreciprocal, thereby obviating any difference in received power.

The antenna 802 has a resonance frequency about 20 kHz lower than theantenna 801. A tuning loop 803 is used to shift the antenna of thetuning loop 802 to match the resonance of the transmitting receiverantenna 801.The signal is coupled to coupling loop 804 and to load 805.

In operation, an RF generator is used to create a 13.56 MHz continuouswave signal. An amplifier 205 provides a 50 dB amplification to producethe maximum power output of 25 W at 206. For purposes of the test, ananalog power meter is used. The power is provided to a coupling loop 220which is adjacent to and wirelessly coupled to the antenna 801 which isformed of a loop 250; and a capacitor 252 that brings the loop toresonance at 13.56 MHz.

FIG. 3 shows the receiver, including the receive loop 102 formed of aninductive loop 113 and capacitor 114, the tuning loop 103, and thecoupling loop 320 which receives the power. A digital power meter 330tests the amount of power that is received after attenuation by a 20 dBattenuator.

The receive side resonator loop combined with tuning loop acts like a1:1 transformer with a low but adjustable coupling factor. The couplingfactor is the distance between the main loop and tuning loop. A tuningloop may be considered as a secondary that produces a short circuit. Theshortcut reduces the overall inductance of the resonator by a smallfraction depending on the coupling factor thus increasing its resonancefrequency without substantially decreasing the quality factor. The mainloop 102 and 103 may be connected to a carriage shown as 333, which canmove the two loops relative to one another. If a resonator which has alow inductance to capacitance ratio is used, it can be extremelyeffective.

FIG. 4 illustrates the received power over specified distances.According to this test, the distance was varied from 1.6 m to 4 m.Distances closer than 1.6 m were not measured, since the closerdistances can cause detuning of the system. Hence, these values areinterpolated, to avoid the detuning effects. Transition from near fieldinto the far field occurs at about 3½ m at 13.56 MHz. This distancechanges the preferred orientation from coaxial to coplanar, therebyaffecting significantly the amount of power that can be received. FIG. 4shows that at 3.5 m, the received power approaches 0 W because of thisorientation variance.

At distances greater than 1.7 m, the calculated distance is closelyrelated to the computed distance.

FIG. 5 illustrates the maximum transferable power. These antennas arehighly linear, meaning that if the transmitter power is doubled, thereceive power will also be doubled. The transmit loop is limited only bythe voltage and current ratings of the capacitors; provided that thereis sufficient cooling. The 30 mm copper loop uses a 200 pF capacitorwith a limit of 9 kV peak and 100 amps carrying. That provides atransmit power of about 300 W.

Because the system is linear, FIG. 5 shows data point scale to atransmit power of 300 W. This shows that the existing system cantransfer 67 W at a distance of 1.6 m. The maximum radiation exposurelimits recommended by ICNIRP would be exceeded by these levels. Thetransfer efficiency, however, is shown in FIG. 6, illustrating that thetransfer efficiency is −15 dB for all distances less than 2½ m. FIG. 7normalizes this distance to the antenna diameter. The tests performed byMIT are also shown in FIG. 7.

Conclusions are as follows. Except for the region closest to the nearfield border and at close distances, the antennas are highly linear, andpower can simply be doubled to double the received power.

The system can operate with a transmit power of 25 W and a transferefficiency of 25% over a distance of 1.5 m. The system is extremelystable with respect to resonance frequency and Q factor. The system canalso power up to 70 W at a distance of 1.5 m. Extrapolation to smallerdistances can also be possible.

Although only a few embodiments have been disclosed in detail above,other embodiments are possible and the inventors intend these to beencompassed within this specification. The specification describesspecific examples to accomplish˜more general goal that may beaccomplished in another way. This disclosure is intended to beexemplary, and the claims are intended to cover any modification oralternative which might be predictable to a person having ordinary skillin the art. For example, other sizes, materials and connections can beused. Although the coupling part of the antenna is shown as a singleloop of wire, it should be understood that this coupling part can havemultiple wire loops. Other embodiments may use similar principles of theembodiments and are equally applicable to primarily electrostatic and/orelectrodynamic field coupling as well. In general, an electric field canbe used in place of the magnetic field, as the primary couplingmechanism.

Also, the inventors intend that only those claims which use the-words“means for” are intended to be interpreted under 35 USC 112, sixthparagraph. Moreover, no limitations from the specification are intendedto be read into any claims, unless those limitations are expresslyincluded in the claims.

Where a specific numerical value is mentioned herein, it should beconsidered that the value may be increased or decreased by 20%, whilestill staying within the teachings of the present application, unlesssome different range is specifically mentioned. Where a specifiedlogical sense is used, the opposite logical sense is also intended to beencompassed.

1. A wireless power system, comprising: a signal generator, having aconnection to a source of power, and which creates a modulated signal ata first frequency; a transmitting antenna, transmitting a magnetic fieldhaving said first frequency and based on power created by said signalgenerator; a receiving antenna, receiving a magnetic power signalcreated by said transmitting antenna, said receiving antenna being adistance greater than 1 m spaced from said transmitting antenna; and aload receiving part, receiving power from said receiving antenna;wherein a transfer efficiency between said transmitting antenna and saidreceiving antenna is greater than 25% at 1 m of distance between saidtransmitting antenna and said receiving antenna.
 2. A system as in claim1, wherein said transmitting antenna transmits a power of 25 W.
 3. Asystem as in claim 1, wherein said transfer efficiency is greater than25% at 1.5 m distance between said transmitting antenna and said receiveantenna.
 4. A system as in claim 1, wherein said transmitting antenna isa capacitively coupled dipole, and said receive antenna is acapacitively coupled dipole.
 5. The system as in claim 1, furthercomprising a coupling loop, on the transmitting antenna, coupleddirectly to said signal generator and said transmitting antenna, andunconnected by any wire connection to said transmitting antenna.
 6. Thesystem as in claim 5, further comprising a coupling loop, on thereceiving antenna, coupled between said receiving electronics and saidantenna, such that said receiving electronics are not directly connectedby any wire to said receiving antenna.
 7. The system of claim 1, furthercomprising a tuning loop, movable relative to said receiver, and saidmovement effecting a resonant frequency of said receiver.
 8. A method oftransmitting power wirelessly, comprising: creating a modulated signalat a first frequency based on power from a power source; using atransmitting antenna to transmit a magnetic field having said firstfrequency and based on said power from said power source; wirelesslyreceiving the magnetic field created by said transmitting antenna at adistance greater than 1 m spaced from said transmitting antenna; andcoupling power from said wirelessly receiving, to a load, with atransfer efficiency between said transmitting antenna and said receivingantenna of greater than 25%.
 9. A method as in claim 8, wherein saidtransmitting antenna transmits a power of 25 W.
 10. A method as in claim8, wherein said transfer efficiency is greater than 25% at 1.5 mdistance between said transmitting antenna and said receive antenna 11.A method as in claim 8, wherein said transmitting antenna is acapacitively coupled dipole, and said receive antenna is a capacitivelycoupled dipole.
 12. The method as in claim 8, further comprising using acoupling loop, on the transmitting antenna between said RF generator andsaid transmitting material, such that said RF generator is not directlyconnected by a wire to said transmitting antenna.
 13. The method as inclaim 12, further comprising using a coupling loop, on the receivingantenna, coupled between said receiving electronics and said antenna,such that said receiving electronics are not directly coupled to saidreceiving antenna.
 14. The method of claim 8, further comprising movinga tuning loop, movable relative to said receiver, wherein a resonantfrequency of said receiver is changed by said moving.
 15. A wirelesspower system, comprising: a signal generator, having a connection to asource of power, and which creates a modulated signal at a firstfrequency; a transmitting antenna, transmitting a magnetic field havingsaid first frequency and based on power created by said signalgenerator; a receiving antenna, receiving a magnetic power signalcreated by said transmitting antenna, said receiving antenna being adistance greater than 1 m spaced from said transmitting antenna; and aload receiving part, receiving power from said receiving antenna;wherein said load receives a power of at least 2½ watts at a distance of1½ meters from said transmitting antenna.
 16. A system as in claim 15,wherein said transmitting antenna transmits a power of 25 W.
 17. Asystem as in claim 15, wherein said transmitting antenna is acapacitively coupled dipole, and said receive antenna is a capacitivelycoupled dipole.
 18. The system as in claim 15, further comprising acoupling loop, on the transmitting antenna, coupled directly to saidsignal generator and said transmitting antenna, and unconnected by anywire connection to said transmitting antenna.
 19. The system as in claim15, further comprising a coupling loop, on the receiving antenna,coupled between said receiving electronics and said antenna, such thatsaid receiving electronics are not directly connected by any wire tosaid receiving antenna.
 20. The system of claim 15, further comprising atuning loop, movable relative to said receiver, and said movementeffecting a resonant frequency of said receiver.